This invention relates to a magnetic recording medium, and more specifically to such a medium with good running quality and surface properties.
Generally, the magnetic recording medium is made by coating polyester film or other base with a magnetic coating material which consists of magnetic particles such as powdered .gamma.-Fe.sub.2 O.sub.3 dispersed in macromolecular resin binder. The nature of the magnetic coating used has important bearings not only upon the running quality and other physical properties but also upon the magnetic characteristic of the medium that is related to the dispersibility of the magnetic particles and other factors, and this nature largely depends on the binder and other additives employed. Also, the physical and electromagnetic properties of the product are closely interrelated. A variety of binders for magnetic coating materials have hitherto been introduced, but none have proved fully satisfactory.
Ordinarily, for the preparation of magnetic coating materials, good dispersibility is hardly attained because the powdered magnetic material, such as .gamma.-Fe.sub.2 O.sub.3, are generally hydrophilic while the lacquers for dispersing them are lipophilic (oleophilic). To improve the dispersibility, surface active agents have been extensively used in mixing magnetic powders with lacquers or resin coating materials. With such an agent the dispersion effect is obtained with relative ease.
It is well-known that curable resins as binders can impart good physical and electrical properties to magnetic coating materials. When such a magnetic coating material is cured by the addition of a polyfunctional aromatic isocyanate, the additional use of a surface active agent as a dispersant will bring various drawbacks. This is because the surface active agent will prematurely react with the isocyanate before the latter reacts with the active radical of the resin, with the consequence that the inadequate curing of the binder confers unsatisfactory physical properties on the resulting recording medium such as a magnetic tape. In particular, the lubrication effects (low friction and high running quality) and surface toughness (low abrasion) which are otherwise given by the curable binder will be insufficient.